Fuel cell systems are energy conversion systems for supplying a fuel gas and an oxidizing gas to a membrane electrode assembly and generating an electrochemical reaction, so as to convert chemical energy to electrical energy. Among the fuel cell systems, a solid polymer electrolyte fuel cell stack in which a solid polymer membrane is used as an electrolyte is expected to be used as an in-vehicle power source system since the solid polymer electrolyte fuel cell stack can be miniaturized at low cost and has a high output density.
In the fuel cell stack, hydrogen ion produced in an anode moves through an electrolyte membrane to a cathode while being hydrated. Thus, water is insufficient in the vicinity of a surface in the anode side of the electrolyte membrane, which becomes a dry state. In order to continue power generation of the fuel cell stack, it is necessary to supply water to the anode side. The electrolyte membrane for use in the solid polymer electrolyte fuel cell stack has good hydrogen ion conductivity when in a moderately humid state. However, when its water content is lowered, an electrical resistance of the electrolyte membrane increases excessively to prevent the electrolyte membrane from functioning properly.
A method of humidifying a reaction gas by using a humidifying device is well known as the method of supplying water to the electrolyte membrane. For example, Japanese Patent Laid-Open No. 2005-116368 discloses a fuel cell system for humidifying an oxidizing gas by a humidifying device in which a hollow fiber membrane bundle comprising a plurality of hollow fiber membranes is accommodated. A high-humidity oxidizing off-gas including a large amount of water produced by a cell reaction flows inside the hollow fiber membrane, and a low-humidity oxidizing gas introduced from the atmosphere flows outside the hollow fiber membrane. A moisture exchange is carried out between the oxidizing off-gas and the oxidizing gas to humidify the oxidizing gas.
Patent Document 1: Japanese Patent Laid-Open No. 2005-116368